The present invention is directed to a method and a device for detecting stenoses in a tubular line system during an extracorporeal hemotherapy.
Extracorporeal hemotherapy is a standard procedure today which is used, above all, for treating renal insufficiency in the form of hemodialysis, hemofiltration, or hemodiafiltration. An extracorporeal hemotherapy requires effective access to the patient""s circulatory system. For some years now, to access the patient""s blood, it has proven effective to position a shunt underneath the skin, between an artery and a vein. During extracorporeal hemotherapy, the patient""s blood flows via the arterial branch of the tubular line system into the hemotherapeutic unit, for example into a hemodialyzer or hemofilter, and from the hemotherapeutic unit via the venous branch of the line system back to the patient. The blood is delivered by a volumetric blood pump, in particular a roller pump positioned in the arterial branch of the line system. The known protection systems generally monitor the pressure of the blood, both in the arterial branch, as well as in the venous branch of the line system. To accomplish this, provision is made for an arterial pressure sensor upstream from the blood pump, and for a venous pressure sensor downstream from the hemotherapeutic unit.
In practical applications, it does occasionally happen that the blood tube forms a kink between the arterial blood pump and the dialyzer. One particularly endangered spot is directly before the inlet to the dialyzer. The kink formation leads to a constriction (stenosis) in the extracorporeal circulation circuit, where a substantial pressure difference builds up. The blood is forced at a high speed through the stenosis. The extremely large speed gradients can lead to shearing stresses that the erythrocytes are no longer able to withstand; the result is hemolysis. If a kink formation of this kind goes unnoticed for an extended period of time, the hemolysis can result in life-threatening health impairments.
It is true that the venous pressure sensor detects a complete closure of the blood tube, however, existing protective systems are only able to a limited extent to detect a constriction that is still partially permeable. A stenosis upstream from the hemotherapeutic unit does, in fact, lead to an increase in the pressure at the outlet of the blood pump, however, the conveying capacity of the latter remains virtually constant within a broad range. Thus, the pressure values do not change significantly at the arterial pressure sensor, nor at the venous pressure sensor. The latter depend essentially only on the delivery rate and the resistances to flow in the entry to and/or return from the patient.
One reliable method for detecting a kink location in the line system would be to introduce an additional pressure sensor at the outlet of the arterial blood pump. However, this approach would entail a considerably costlier effort with respect to the tubular systems. It would require additional connectors having hydrophobic filters, etc.
The German patent DE 199 01 078 describes a device for detecting stenoses during extracorporeal hemotherapy, which makes use of the arterial pressure sensor. The device is based on the principle of the blood pump generating an oscillating pressure signal that propagates across the tubular line system. In response to a serious enough stenosis between the blood pump and the hemotherapeutic unit, the pressure rises so markedly downstream from the blood pump that the occlusion of the pump rollers is partially removed. As a result, at times during the pumping operation, a little blood flows back upstream into the line system from the blood pump, causing an increase in the pulse amplitude measured at the arterial pressure sensor. At the same time, the generally negative arterial mean pressure approaches the zero line, since the delivery capacity abates.
In practice, however, the above method is only conditionally suited for detecting a beginning stenosis, since the occlusion of the pump does not subside until a delivery pressure of approximately 2 bar is attained. Until then, no significant change in the pressure signal is observed at the arterial pressure sensor. Nevertheless, the high pressure gradients at the stenosis can already lead to a dangerous hemolysis.
From the International Patent Application No. WO 97/10013, a method is known for detecting a stenosis at the entry to the patient, where an oscillating pressure signal, transmitted via the tubular line system, is analyzed. The pump signal that is attributed to the rotation of the blood pump is extracted from the pressure signal to facilitate detection of the patient""s pulse signal, which is utilized for identifying the stenosis.
The object of the present invention is to provide a method for detecting stenoses, which is easily implemented in known hemotherapeutic devices, and which is distinguished by a high sensitivity such that stenoses may be detected before changes in the average value and amplitude of the arterial and venous pressure signal become discernible. It is also an object of the present invention to provide a device for detecting stenoses according to this method.
The method and the device of the present invention utilize the change in the dynamic performance of the tubular line system in response to the occurrence of a stenosis. The cause of the change in the dynamic performance is the compliance of the line system, i.e., the elastic yielding under pressure. This compliance is not regarded as a disturbance factor, rather, it is selectively utilized for detecting stenoses.
Based on the change in the dynamic performance, a stenosis that is beginning to form may be reliably detected, before the occlusion of the arterial blood pump is removed. One may make use of the tubular line systems (disposables) of the known hemotherapeutic devices, without the need for relying upon additional sensors or the like.
The method according to the present invention provides for an analysis of the frequency spectrum of the oscillating pressure signal. When a change in the frequency spectrum occurs, a stenosis is inferred. It is attributed to a change in the dynamic response of the tubular line system caused by the constriction.
For the method of the present invention, it is unimportant how the oscillating pressure signal is generated in the tubular line system. The volumetric blood pump, in particular the roller pump, which is used to deliver the blood in the arterial branch, has pressure pulses, which are preferably measured as oscillating signals.
The change in the dynamic response of the tubular line system results in a variable attenuation of the oscillating pressure signal. A stenosis leads to a heavy attenuation, particularly in response to higher frequencies.
An analysis, preferably a Fourier transform of the oscillating pressure signal, is carried out, and the attenuation of at least one harmonic component of the pressure signal is determined. From the change in the attenuation, the existence of a stenosis is then concluded. A stenosis may be reliably detected when only the attenuation of the first harmonic component is ascertained. However, the attenuation of one or of a plurality of harmonic component(s) of a higher order than the first harmonic component may also be determined. In principle, it is possible to infer a stenosis when an attenuation relating to only one harmonic component is present, or when an attenuation relating to a plurality of harmonic components is present. In this context, the known statistical methods may be utilized to further enhance sensitivity or reliability.